Radio Frequency identification (RFID) systems are used for identification and/or tracking of equipment or inventory such as pallets, trucks, dollies or boxes or even the whereabouts of some animinals, such as livestock incertain situations. RFID systems are radio communication systems in which communications is provided between a radio transceiver, or interrogator, and a number of small, identifying labels or tags. These tags are read while in the radiation pattern or field of the interrogator, which is connected to a computer-based tracking system. The intent of an RFID system is to provide a reliable and secure architecture that meets a predetermined performance requirement, while minimizing the cost of the interrogator and the tags. In the operation of RFID systems, the interrogator transmits to the tags using modulated radio signals, and the tags respond by transmitting modulated radio signals back to the interrogator. Specifically, the interrogator first transmits an amplitude modulated signal to the tag. Next, the interrogator transmits a continuous-wave (CW) radio signal to the tag. The tag then modulates the CW signal using modulated back scattering (MBS) wherein the antenna is electrically switched, by the tag's modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation; thereby encoding the tag's information onto the CW radio signal. The interrogator demodulates the incoming modulated radio signal and decodes the tag's information message.
The tags may be used in time-critical applications where a short message or small amount of information must be exchanged between the interrogator and the tags. One example of a time-critical application is where the tag, or multiple tags, appear within the field of the interrogator for a short period of time. When the small amounts of information must be exchanged, multiple tags easily may be read while within the field of the interrogator. When large amounts of information must be exchanged, however, generally only one tag may be read while within the field of the interrogator. Thus, it is desirable to be able to selectively exchange data between a designated tag and the interrogator. In addition, it is desirable to be able to periodically communicate with other tags which reside in the field or enter the field while the long data transfer with a tag is ongoing so that these other tags do not corrupt the data transfer in progress.